anatomy and physiology of large bowel Flashcards
what are the 3 main functions of the colon
- finish absorption of nutrients + water
- synthesize certain vitamins (vit K etc.)
- form + eliminate faeces
what characteristic feature of the GI tract is absent in the colon
villi
where does the colon extend from
appendix to anus
why is small bowel obstruction more common than large bowel obstruction
small bowel is half the diameter of the large bowel
what is the caecum
a sac-like structure that is suspended inferior to the ielocaecal valve (R side) and it receives the contents of the ileum
what type of tissue is contained in the appendix and what does this suggest about its function
lymphoid tissue - suggest immunological function
what is the associations of UC and crohns with appendicectomy
reduced risk of UC but increased risk of crohns
what parts of the colon are retroperitoneal
ascending and descending colon
what parts of the colon does the mesocolon tether to the posterior abdo wall
transverse and sigmoid
what are taeniae coli
three bands of smooth muscle that make up the longitudinal muscle layer of the muscularis of the large intestine (apart from the terminal end)
what are the haustra
pouches formed by the contractions of taeniae coli causing the colon to bunch up
what are epiploic appendages
small fat-filled sacs of visceral peritoneum attached to the taeniae coli
where in the large bowel are taeniae coli and haustra not present
the rectum - here there are well developed layers of muscularis that create strong contractions required for defecation
at what vertebral level is the rectum
S3
what are the rectal valves + function
3 lateral bends that create a trio of internal transverse folds that support the weight of faecal matter and help to separate faeces from gas so that they are not passed simultaneously
where is the anal canal located
in the perineum, completely outside the abdominopelvic cavity
what is the internal anal sphincter muscle
a thickened extension of the circular SMOOTH muscle layer surrounding the colon -> contractions are involunatry
what is the external anal sphincter
a flat plane of SKELETAL muscle that is under voluntary control
what kind of cells connects the anal canal to the skin on the outside
stratified squamous epithelial mucosa - these cells can withstand the high levels of abrasion as faeces pass through
what are anal columns
longitudinal folds of the mucous membrane that each houses a grid of arteries and veins
what is an anal sinus
the depression between the anal columns -> secretes mucus that facilitates defecation
what is the dentate line (anus)
aka the pectinate line -> a horizontal, jagged band that runs circumferentially just below the level of the anal sinuses and represents the junction between the hind gut and external skin
why is there a difference in sensation on either side of the dentate line (anus)
the mucosa above is faily insensitive as it is innervated by visceral sensory fibres;
the mucosa below is very sensitive as it is innervated by somatic sensory fibres
differences between the walls of the large and small intestines (2)
- no villi or circular folds in large
- less enzyme secreting cells in large
what cells is the mucosa of the colon primarily made up of (3)
- simple columnar epithelium
- enterocytes (absorptive)
- goblet cells
what commonly (unconciously) ingested substance strips the muscous layer from the gut
detergent -> may contribute to the development of IBD
what is the role of mucous in the colon (2)
- eases the movement of faeces
- protects the intestine from the effects of the acids and gases produced by enteric bacteria
what do the enterocytes absorb in the colon (3)
- water
- salts
- vitamins produced by intestinal bacteria
what do the normal colonic bacterial flora aid with (2)
- facilitate chemical digestion + absoprtion
- synthesize certain vitamins (vit B7 (biotin), vitamin K, vit B5 (Pantothenic acid))
how are bacteria prevented from crossing the mucosal barrier in the gut
- peptidoglycan in the bacteria cell walls activate the release of chemicals by the epithelial cells which attract immune cells into the mucosa
- dendritic cells open the tight junctions between the epithelial cells and extend probes into the lume to evaluate the antigens
- APC dentritic cells travel to neighboring lymphoid tissue
- T cells detect the antigens and trigger an IgA mediated response
how much liquid is resorbed in the small bowel a day
around 8L (of the 9L in the GI tract)
what regulates the movement of chyme into the caecum
the ileocaecal sphincter
what tiggeres colon movements (mechanical digestion)
distention of the caecum
what is the action of fibre (2)
- softens stool
- increases the power of colonic contractions, optimising the activities of the colon
what part of the colon d haustral contractions generally occur
the transverse and descending colon
what type of movement is seen in haustral contractions
sluggish segmetation
what causes chyme movement through the haustra
distension of a haustrum with chyme causes the muscle to contract and residue to be pushed into the next haustrum
how frequently do haustral contractions occur
every 30min, each lasting 1 min
3 components to mechanical digestion
- haustral contactions
- peristalsis
- mass movement
what is mass movement (mechanical digestion)
strong waves starting midway through the transverse colon and quickly force the contents towards the rectum
how frequently do mass movements occur (digestion)
3-4 times a day, either while eating or immediately after
what is the gastrocolic reflex
distension of the stomach and the breakdown products in the small intestine trigger this reflex to increase motility -> mass movements
what is responsible for chemical digestion in the large intesinte
solely bacteria -> no digestive enzymes are secreted
what is saccharyoltic fermentation
when bacteria breakdown remaining carbs, discharging hydrogen, CO2 and CH4 gases that create flatusint he colon
when is flatus production increased
when foods rich in otherwise indigestible (but the bacteria can digest them) surgars and complex carbs e.g. beans
what is faeces formed up of (6)
- undigested food residues
- unabsorbed digested substances
- bacteria
- old epithelial cells
- inorganic salts
- water (to let it pass smoothly out of the body)
how is the defecation reflex provoked
stretching of the rectal wall due to movement of faeces from the colon into the rectum (mass movement(
what mediates the defecation reflex
this is a parasymp reflex mediated by the spinal chord
what occurs in the defecation reflex (4)
- contraction of the colon and rectum
- relaxation of the internal sphincter
- initally - contraction of the external sphincter
- voluntary relaxation of the external sphincter
describe the voluntary control of the defecation reflex
presence of faeces in the anal canal sends a signal to the brain indicating the need to deficate -> the external sphincter muscle can be voluntarily controlled so that defecation occurs at an appropriate time
what will delayed defecation lead to (in terms of the stool)
increased water absorption leading to firmer stool -> potentially leading to constipation
how does diarrhorea occur
waste material moves too quickly through the intestines and so not enough water can be absorbed -> can be a result of ingested pathogens
what chart is used to describe stool
bristol stool chart
how may types of stool are on the bristol stool chart
7 -> 1 is hard to pass and 7 is entirely liquid
